(a) Field of the Invention PA1 (b) Description of the Prior Art
This invention relates to an oblique viewing optical system for endoscopes and more particularly for endoscopes for inspecting infants.
A conventional oblique viewing optical system of this kind is disclosed, for example, in Japanese Patent Publication No. Sho 58-56848. As shown in FIG. 1, this is formed of an objective 1, a relay lens 2 connected to the objective 1, a field lens 3 and an eyepiece 4, the objective 1 is formed of a front lens group L.sub.1 comprising in combination an oblique plano-concave lens 5, a visual field direction changing prism consisting of a first prism 6 and second prism 7 and a plano-convex lens 8, and a rear lens group L.sub.2 comprising a bi-convex lens 9 and plano-concave lens 10 cemented to each other so that a light incident obliquely to the axial direction of the endoscope will once pass through the boundary of the first prism 6 and second prism 7, then will be reflected in turn by the first reflecting surface (bottom surface) and second reflecting surface (boundary surface with the first prism 6) of the second prism 7 and will be directed to the direction coinciding with the axial direction. However in the case of this oblique viewing optical system, there has been a problem that, as the first reflecting surface of the second prism 7 is at a considerably large angle with the axial direction of the endoscope, the contour of the second prism 7 will be so small that it will be very difficult to work the second prism 7 and to assemble the objective 1 in view of the fact that the diameter of an ordinary endoscope for inspecting infants is only about 1 mm. There has been also a problem that the length of the second prism 7 is so small that the deviation in the visual field direction by the position deviation will be likely to become large. Further, there has been a problem that, as the width of the rear end surface of the second prism 7 can not help becoming small, in the case of an optical system of a wide (bright) light bundle, the light bundle will be cut and the light amount loss will be large. Also, in the case of this oblique optical system, an air layer 11 is provided on the boundary between the first prism 6 and second prism 7 so that the light may be transmitted and reflected on this boundary by utilizing the refractive index difference between the air layer 11 and the prisms 6 and 7. However, there has been a problem that it is so difficult to provide the air layer 11 in the front lens group L.sub.1 of the extremely small objective 1 as described above that, as a result, it will be more difficult to assemble the objective 1.